Vehicle powder coating system

ABSTRACT

An apparatus for applying powder coating material onto large objects such as automotive, truck or other vehicle bodies includes a powder spray booth defining a controlled area within which to apply powder coating material onto the vehicle bodies, a powder kitchen located at a remote position from the powder spray booth, and, a number of feed hoppers located proximate the booth which receive powder coating material from the powder kitchen and supply it to automatically or manually manipulated powder spray guns associated with the booth. Oversprayed powder coating material is removed from the booth interior by a powder collection and recovery system which transmits the oversprayed powder back to the powder kitchen for recirculation to the powder spray guns.

This is a continuation of U.S. patent application Ser. No. 11/167,830,filed Jun. 27, 2005 now U.S. Pat. No. 7,166,164, for VEHICLE POWDERCOATING SYSTEM, which is a continuation of U.S. patent application Ser.No. 10/752,809, filed Jan. 7, 2004, now U.S. Pat. No. 6,929,698, forVEHICLE POWDER COATING SYSTEM, which is a continuation of U.S. patentapplication Ser. No. 10/041,201, filed Jan. 8, 2002, now U.S. Pat. No.6,679,193, for VEHICLE POWDER COATING SYSTEM, which is a divisional ofU.S. patent application Ser. No. 09/425,786, filed Oct. 22, 1999, nowU.S. Pat. No. 6,361,605, for VEHICLE POWDER COATING SYSTEM, which is adivisional of U.S. patent application Ser. No. 08/862,934, filed May 29,1997, now U.S. Pat. No. 6,071,558, for VEHICLE POWDER COATING SYSTEM,which is a divisional of U.S. patent application Ser. No. 08/320,921,filed Oct. 11, 1994, now U.S. Pat. No. 5,743,958, for VEHICLE POWDERCOATING SYSTEM, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/066,873, filed May 25, 1993, now abandoned, forVEHICLE POWDER COATING SYSTEM, the entire disclosures of which are herbyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to powder coating systems, and, moreparticularly, to a powder coating system for use in vehiclemanufacturing facilities including a powder spray booth, a powdercollection and recovery system, and a powder supply which transmitsvirgin powder coating material and a mixture of virgin and reclaimed oroversprayed powder coating material from a remote location to coatingdispensers associated with the spray booth.

BACKGROUND OF THE INVENTION

The application of coating materials to large objects such as automotiveand other vehicle bodies has conventionally been accomplished in spraybooths having an elongated tunnel-like construction formed with an inletfor the ingress of a vehicle body, a coating application area, a curingor drying area in some designs, and, an outlet for the egress of thevehicle body. In many systems, “conditioned” air, i.e. humidified andfiltered air, is introduced by a blower or feed fan into a plenumchamber at the top of the spray booth and then directed downwardlytoward the vehicle body moving through the booth. The conditioned airpicks up oversprayed coating material within the booth interior and thisair entrained oversprayed material is drawn downwardly through the flooror side of the booth by one or more exhaust fans. Filters are providedto capture the oversprayed coating material, and the resulting filteredor clean air is withdrawn from the booth and either exhausted toatmosphere or recirculated within the system for reuse.

The coating material in most common use for vehicles such asautomobiles, trucks and the like is a high solids, resinous paintmaterial which contains a relatively high percentage of liquid solventcomponents to facilitate atomization of the resinous material. Theproblems attendant to the recovery of oversprayed, resinous paintmaterial have been well documented and present a continuingenvironmental problem for the coating and finishing industry. See U.S.Pat. No. 4,247,591 to Cobbs, et al. and U.S. Pat. No. 4,553,701 toRehman, et al.

As disclosed in U.S. Pat. No. 5,078,084 to Shutic, et al., owned by theassignee of this invention, powder coating material has been suggestedas an alternative to solvent based liquid paint materials for thecoating of large objects such as vehicle bodies. In the practice ofpowder coating, a powdered resin is applied to the substrate and thenthe substrate and powder are heated so that the powder melts and whensubsequently cooled, forms a solid continuous coating on the substrate.In most powder spraying applications, an electrostatic charge is appliedto the sprayed powder which is directed toward a grounded object to becoated so as to increase the quantity of powder which attaches to thesubstrate and to assist in retaining the powder on the substrate. Theapplication of powder material onto automotive or truck bodies isperformed in a spray booth which provides a controlled area whereinoversprayed powder which is not deposited on the vehicle body can becollected. Containment of the oversprayed powder within the booth isaided by an exhaust system which creates a negative pressure within thebooth interior and causes the oversprayed powder to be drawn through thebooth and into a powder collection and recovery system. The recovered,oversprayed powder can be saved for future use, or is immediatelyrecycled to powder spray guns associated with the powder spray booth.

A number of problems are inherent in coating automotive and othervehicle bodies with powder coating material. Due to the design ofvehicle manufacturing facilities, the source of coating material isusually positioned at a remote location from the spray booth, i.e. asmuch as several hundred feet. Moreover, large quantities of powdercoating material, e.g. on the order of 300 pounds per hour and up, mustbe transferred from the source to the spray booth over this relativelylong distance at flow rates such as 1 to 2 pounds per second.Additionally, the powder coating material must be transferred with theappropriate density and particle distribution in order to obtain anacceptable coating of the powder material on the vehicle bodies. Theterm “density” refers to the relative mixture or ratio of powder-to-air,and the term “particle distribution” refers to the disbursion of powderparticles of different sizes within the flow of air entrained powdermaterial to the spray guns associated with the powder spray booth. Ithas been found that currently available powder coating systems aregenerally incapable and/or deficient in transporting large quantities ofpowder material at high flow rates over long distances, whilemaintaining the desired density and particle distribution.

As noted above, not all of the powder coating material discharged withinthe powder spray booth adheres to the vehicle bodies movingtherethrough. This oversprayed powder material is collected by a powdercollection and recovery system at the base of the booth as disclosed,for example, in U.S. Pat. No. 5,078,084 to Shutic, et al. In systems ofthis type, the powder collection and recovery system includes individualgroups or bank of cartridge filters each contained within a series ofindividual powder collection chambers mounted side-by-side beneath thefloor of the spray booth. A single exhaust fan or blower creates anegative pressure within the booth interior, which draws oversprayed,air entrained powder material into each of the individual powdercollection chambers where the powder is collected on the walls of thecartridge filters and “clean air” passes therethrough for eventualdischarge to atmosphere. Reverse air jets are operated to dislodge thecollected powder from the walls of the cartridge filters which thenfalls to the base of the powder collection chambers where it is removedfor collection or recirculation back to the spray guns associated withthe powder spray booth.

In high volume applications such as coating automotive vehicle bodies,serviceability of the powder collection and recovery system, and, theapplication of a uniform negative pressure within the booth interior areof particular concern. It has been found somewhat difficult in certaininstances to obtain a uniform negative pressure within the boothinterior using a single exhaust or blower fan, which, in turn, adverselyaffects the efficiency with which the powder coating material can becollected and also can disrupt the pattern of powder coating materialdischarged from the spray guns onto the vehicle bodies moving throughthe booth. There has also been a need in systems of this type to improvethe serviceability of the reverse air jet valves and cartridge filterscontained within each powder recovery chamber.

An additional problem with powder coating systems of the type describedabove involves recovery of oversprayed powder for recirculation back tothe spray guns associated with the powder spray booth. Virgin powdercoating material contains a wide particle size distribution, i.e. itincludes powder particles which vary substantially in size. The largerpowder particles tend to more readily adhere to an object to be coatedwithin the spray booth because they receive a higher electrostaticcharge due to their size than smaller particles, and because larger,heavier particles have more momentum than smaller particles whendischarged from a spray gun toward an object to be coated. As a result,the oversprayed powder which does not adhere to the object and iscollected for recirculation back to the spray guns contains aproportionately greater percentage of smaller particles than the virginpowder since a greater percentage of larger particles in comparison tosmaller particles have adhered to the object.

It has been found that the stability of operation of a powder coatingsystem is dependent, at least in part, on avoiding a buildup oraccumulation of “fines,” e.g particles having a size of less than about10 microns. The term “stability” as used herein refers to the ability ofthe system to fluidize, transfer and spray powder coating materialwithout problems created by excessive levels of fines. The presence ofexcessive levels of fines within the powder coating material can resultin poor fluidization of the powder, impact fusion, blinding or cloggingof filter cartridges and sieve screens, increased powder buildup oninterior surfaces of the powder spray booth and on spray guns, and, poortransfer efficiency. The term “impact fusion” refers to the adherence ofa powder particle onto a surface as a result of particle velocity asopposed to electrostatic attraction, and “transfer efficiency” is ameasure of the percentage of powder material which adheres to an objectcompared to the total volume of powder sprayed toward the object.

There is essentially no provision in powder coating systems of the typedescribed above to ensure system operating stability when oversprayedpowder material is recirculated back to the spray guns after collection.Although venting units have been employed to remove fines from supplyhoppers and the like, such units are of limited effectiveness and cannotbe relied upon to control with desired accuracy the level or percentageof fines within a given supply hopper.

SUMMARY OF THE INVENTION

It is therefore among the objectives of this invention to provide apowder spraying system for applying powder coating material onto largeobjects such as automotive or other vehicle bodies which is capable oftransmitting large quantities of powder material over long distances atrelatively high flow rates while maintaining the desired density andparticle distribution, which is capable of automatically maintaining theappropriate volume of powder coating material within the systemirrespective of demand, which efficiently collects and recovers largequantities of oversprayed powder for recirculation, which avoids theaccumulation of excessive levels of fines, and, which is comparativelyeasy to service.

These objectives are accomplished in an apparatus for applying powdercoating material onto large objects such as automotive, truck or othervehicle bodies which includes a powder spray booth defining a controlledarea within which to apply powder coating material onto the vehiclebodies, a “powder kitchen” located at a remote position from the powderspray booth, and, a number of feed hoppers located proximate the boothwhich receive powder coating material from the powder kitchen and supplyit to automatically or manually manipulated powder spray guns associatedwith the booth. Oversprayed powder coating material is removed from thebooth interior by a powder collection and recovery system whichtransmits the oversprayed powder back to one or more mixing hopperswithin the powder kitchen for recirculation to the powder spray guns.

One aspect of this invention is predicated upon the concept of providingan efficient means for the transfer of powder coating material from aremote location, i.e. at the powder kitchen, to the feed hoppers locatedproximate the spray booth. This is accomplished in the apparatus of thisinvention by a powder transfer system which is operated using vacuum ornegative pressure instead of positive pressure. The powder kitchen,includes one or more primary hoppers each coupled to a powder receiverunit connected to a source of virgin powder coating material within thepowder kitchen. A transfer line interconnects the primary hopper with apowder receiver unit associated with each of the feed hoppers at thespray booth. A first vacuum pump is operative to create a negativepressure within the powder receiver unit associated with the primaryhopper to draw virgin powder material from the source into the powderreceiver unit which, in turn, supplies powder to the primary hopper. Asecond vacuum pump applies a negative pressure within each powderreceiver unit associated with the feed hoppers so that virgin powdermaterial from the primary hopper located in the powder kitchen is drawnthrough the long transfer line into the powder receiver units associatedwith the feed hoppers in the vicinity of the spray booth. The powderreceiver units at the spray booth fill their respective feed hopperswith powder, which, in turn, is transferred from the feed hoppers bypowder pumps to powder spray guns within the spray booth.

This same principal of powder transfer under the application of negativepressure is employed in the collection of oversprayed powder materialfrom the spray booth. A reclaim hopper located in the powder kitchen iscoupled to a powder receiver unit connected by a reclaim line to thepowder collection and recovery system associated with the powder spraybooth. A vacuum pump creates a negative pressure within the powderreceiver unit associated with the reclaim hopper which receivesoversprayed powder from the booth, and, in turn, transfers suchoversprayed powder to the reclaim hopper. In one presently preferredembodiment, this reclaimed, oversprayed powder is then transmitted fromthe reclaim hopper under the application of negative pressure by anothervacuum pump to supply the oversprayed powder to powder receiver unitsconnected to feed hoppers located near the booth. These feed hoppersthen supply the oversprayed powder to spray guns associated with thespray booth which are operative to apply the powder to other portions ofthe vehicle body being coated.

In an alternative embodiment, the reclaim hopper and primary hopper areeach connected to a mixing hopper located within the powder kitchen.Powder pumps within the primary and reclaim hoppers transfer a selectedratio of virgin powder and reclaim or oversprayed powder into the mixinghopper where such powders are intermixed in preparation for transfer tospray guns associated with the spray booth. In accordance with a methodof this invention wherein particle size distribution within the powdercontained in the mixing hopper is mathematically predicted, the supplyof virgin and reclaim powder introduced into the mixing hopper, iscontrolled so that the volume percentage of fines contained within themixing hopper does not exceed a predetermined maximum percentage. Thisensures stable operation of the powder coating system when applyingreclaim or oversprayed powder onto objects within the booth.

It has been found that large quantities of powder coating material, e.g.on the order of 300 pounds per hour and up, can be efficiently andeffectively transmitted by the vacuum transfer system described above tosatisfy the particular demands of automotive manufacturing facilitieswherein the source of the powder coating material is located remote fromthe powder spray booth. It is believed that the use of vacuum, asopposed to positive pressure, uses less air and therefore reduces theoverall energy requirements of the system. Additionally, in the event ofa leak in one of the transfer lines extending between the powder kitchenand spray booth, the powder material is drawn inwardly within suchtransfer lines because of the vacuum therein instead of being forcedoutwardly as would be the case with a positive pressure powder transfersystem. This reduces the risk of contamination of the facility withpowder in the event of a leakage problem.

Another feature related to the powder transfer aspect of this inventioninvolves the automatic monitoring and replenishment of virgin powdercoating material and oversprayed powder material as the coatingoperation proceeds. Each of the primary hoppers, reclaim hoppers andfeed hoppers is carried by a load cell connected to a programmable logiccontroller.

These load cells are set on a zero reference with the empty weight oftheir respective hoppers, and are effective to measure the weight ofpowder material which enters each individual hopper during operation ofthe system. Considering a primary hopper, for example, the load cellassociated therewith sends a signal to the controller indicative of theweight of powder within such primary hopper during operation of thesystem. In the event the quantity of powder material within the primaryhopper falls beneath a predetermined minimum, the controller receives asignal from the load cell and operates the vacuum pump connected to thepowder receiver unit associated with such primary hopper so thatadditional, virgin powder coating material is transmitted from thesource, into the powder receiver unit and then to the primary hopper.Once that primary hopper receives a sufficient level of powder coatingmaterial, further supply of powder is terminated. The reclaim hopper andfeed hoppers operate in the same manner so that appropriate levels ofpowder coating material are maintained in each during a powder coatingoperation. In one embodiment, a connector line is provided between eachprimary hopper and reclaim hopper so that virgin powder coating materialcan be supplied from the primary hoppers to the reclaim hopper in theevent the quantity of oversprayed powder material collected within thepowder collection and recovery system of the spray booth is insufficientto maintain the quantity of powder material within the reclaim hoppersat the desired level.

In an alternative embodiment, the programmable controller governs thetransfer of virgin powder coating material from each primary hopper, andthe transfer of reclaim or oversprayed powder from associated reclaimhoppers, into a mixing hopper in accordance with a selected ratiodetermined by the method noted above and discussed in detail below. Themixing hopper, in turn, feeds a mixture of virgin and reclaim powder toone or more spray guns.

Another aspect of this invention involves the provision of structurewithin each of the primary hoppers, reclaim hoppers and feed hoppers toensure that the powder coating material is transferred within thesystem, and supplied to the spray guns, with the desired density andparticle distribution. In this respect, principals of operation similarto those employed in the powder feed hopper disclosed in U.S. Pat. No.5,018,909 to Crum, et al., owned by the assignee of this invention, areused in the various hoppers of this invention. Generally, each of thehoppers herein include a porous plate which receives an upward flow ofair directed through baffles located within an air plenum in the baseportion of such hoppers. Agitators, including rotating paddles orblades, are located above the porous plate to ensure that the powdermaterial is properly fluidized, has a homogeneous distribution of powderparticles and has the appropriate density or air-to-particle ratio priorto discharge from the respective hoppers.

A still further aspect of this invention is predicated upon the conceptof providing an efficient, easily serviceable powder collection andrecovery system for the powder spray booth, which produces a uniform,downwardly directed flow of air within the booth interior. The powdercollection and recovery system herein is modular in constructionincluding a number of powder collection units mounted side-by-side alongthe length of the powder spray booth beneath its floor. Each of thepowder collection units includes a powder collection chamber housing twogroups or banks of cartridge filters mounted in an inverted V shapeabove an angled, fluidizing plate located at the base of the powdercollection chamber. A limited number of individual powder collectionunits are connected by a common duct to a separate exhaust fan or blowerunit. Each exhaust fan is effective to create a negative pressure withinits associated powder collection units to draw air entrained,oversprayed powder material from the booth interior, downwardly throughthe floor of the booth and then into each of the powder collectionchambers. The oversprayed powder material collects on the walls of thecartridge filters and “clean” air passes therethrough into clean airchambers associated with each powder collection unit. Pulsed jets of airare periodically introduced into the interior of the cartridge filtersfrom air jet valves positioned thereabove to dislodge powder collectedon the walls of the filters which then falls onto the angled fluidizingplate at the base of each powder collection chamber for removal. Eachpowder collection chamber has an outlet connected to a common headerpipe, and a gate valve is positioned, in each of these outlet lines. Thesystem controller is effective to sequentially open and close the gatevalves so that collected powder material is removed from the variouspowder collection units in sequence for transfer to the reclaim hopperassociated with the powder kitchen.

The construction of the powder collection and recovery system hereinprovides a number of advantages. Because a number of exhaust or blowerunits are employed, each associated with a limited number of powdercollection units, a more uniform and evenly distributed, downward flowof air is created within the interior of the powder spray booth alongits entire length. This is an improvement over systems having a singleexhaust fan or blower because it has proven difficult to obtain auniform negative pressure within a spray booth having the extreme lengthrequired to coat large objects such as vehicle bodies with only oneblower unit. Servicing of the powder collection and recovery systemherein is also made much easier than in prior designs. The reverse airjet valves are located at the top of the powder collection units foreasy access, and the cartridge filters are easily removed from thepowder collection chambers by one operator. Removal of powder materialfrom each of the powder collection chambers is also made easier by theangled, fluidizing plate at the base thereof which aids in smoothlytransferring powder out of the chambers. Additionally, the walls of thepowder collection chamber are made sufficiently thin so that they arevibrated when the reverse jets of air are activated to assist in thetransfer of powder onto the porous plate.

DESCRIPTION OF THE DRAWINGS

The structure operation and advantages of the presently preferredembodiment of this invention will become further apparent uponconsideration of the following description, taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a partial schematic view of one embodiment of this inventiondepicting one end of a powder spray booth including feed hoppers and aportion of a powder collection recovery system, and including aschematic depiction of the powder kitchen;

FIG. 2 is an elevational view of a powder receiver unit and primaryhopper contained within the powder kitchen;

FIG. 3 is a plan view of the primary hopper shown in FIG. 1;

FIG. 4 is a cross-sectional view taken generally along line 4—4 of FIG.3;

FIG. 5 is an elevational view in partial cross-section of one embodimentof a feed hopper of this invention;

FIG. 6 is a schematic, partially cut-away view of a robot hopper of thisinvention;

FIG. 7 is a schematic, partially cut-away view of the powder collectionand recovery system herein;

FIG. 8 is an end view of a powder collection chamber;

FIG. 9 is a side view of the powder collection chamber depicted in FIG.8;

FIG. 10 is a view similar to FIG. 1, except of an alternative embodimentof the powder coating system of this invention;

FIG. 11 is an elevational view in partial cross section of analternative embodiment of the feed hopper shown in FIG. 5;

FIG. 12 is a graphical depiction of the particle size distribution, byvolume percent, of virgin particulate powder coating material;

FIG. 13 is a graphical depiction of the particle size distribution, byvolume percent, of reclaimed particulate powder coating material; and

FIG. 14 is a calculated set of curves which graphically depicts thepercentage of particulate powder coating material having a particle sizeof less than 10 microns which is present within powder coating materialhaving different percentages of virgin powder after a given number ofreclaim cycles; and

FIG. 15 is block diagram depiction of measuring and control functionsperformed by the controller employed in the embodiment of this inventionillustrated in FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the FIGS., one embodiment of the powder coating system10 of this invention includes a powder spray booth 12, devices fortransferring powder coating material from a powder kitchen 14 to thebooth 12, and, a powder collection and recovery system 16 associatedwith the booth 12. These system elements are described separately below,including a discussion of the operation of each.

Powder Spray Booth

Referring to FIGS. 1 and 2, the powder spray booth 12 includes a ceiling18, floor 20, opposed side walls 22, 24 and opposed end walls defining abooth inlet 26 and a booth outlet 28. See also FIG. 7. This constructionof spray booth 12 defines an interior 30 forming a controlled area inwhich to apply powder coating material onto objects such as a vehiclebody 32 moved by a conveyor 32 through the longitudinally extendingcenter portion 36 of the spray booth 12. Oversprayed powder materialwhich does not adhere to the vehicle body 32 passes through gratings 38located along the floor 20 of spray booth 12 and into the powdercollection and recovery system 16 described in detail below.

The powder spray booth 12 extends for a substantial longitudinaldistance, and can be provided with a variety of powder spray gunspositioned at different locations therealong so that all areas of thevehicle body 32 are coated with powder coating material in the course ofpassage through the booth interior 30. For purposes of illustration, arobot 40 carrying a spray gun 42 is depicted on one side of the spraybooth 12, and an overhead gun manipulator 44 is illustrated in positionabove the vehicle body 32 carrying spray guns 46. Depending upon thesize of the vehicle body 32, the types of powder coating material to beapplied thereto, the desired areas of coverage on the vehicle body 32and other factors, essentially any number of spray guns manipulatedeither automatically or manually can be provided along the length of thespray booth 12 for covering the vehicle body 32 with powder coatingmaterial. The particular location and operation of such spray guns formsno part of this invention of itself, and is therefore not discussedherein.

In the presently preferred embodiment, the vehicle body 32 is held atground potential by the conveyor 32 and an electrostatic charge isimparted to the powder coating material by the spray guns 42 and 46. Theelectrostatic charge applied to the powder material increases thequantity of powder which adheres to the vehicle body 32, and assists inretaining powder thereon, but a relatively large quantity of powdermaterial is nevertheless “oversprayed”, i.e. fails to adhere to thevehicle body 32. This oversprayed powder must be collected and recoveredin the course of the powder coating operation, as described below.

Powder Coating System of FIG. 1.

An important aspect of this invention involves the structure of system10 for transferring the powder coating material from the powder kitchen14 to the spray booth 12. In many vehicle manufacturing facilities, thepowder kitchen 14 is positioned at a remote location from the spraybooth 12, e.g. several hundred feet away, and a large quantity of powdercoating material must be rapidly transmitted therebetween. Powder flowrates of 1–2 pounds per second, and total demand for powder of 300pounds per hour and up, are not uncommon. The overall configuration ofthe powder transfer system of this invention which is capable ofefficiently and economically satisfying such parameters is describedfirst, followed by a detailed discussion of the various separateelements making up such transfer system.

In the embodiment of FIG. 1, the powder kitchen 14 is essentially aclosed housing (not shown) which is provided with “conditioned” air,i.e. filtered and humidified air, supplied from an air house (not shown)of conventional design. Within the powder kitchen 14 is a source 54housing virgin powder coating material, which is connected by a line 56to a first powder receiver unit 58 described in detail below. The powderreceiver unit 58 is connected to a primary hopper 60, and by a suctionhose 61 to a first vacuum pump 62, both of which are housed in thepowder kitchen 14. The primary hopper 60 is connected by a transfer line64 to a second powder receiver 66 coupled to a first feed hopper 68.This transfer line 64 carries a first gate valve 70, and is connected toa first makeup air valve 72, both located downstream from the primaryhopper 60 and within the powder kitchen 14. The makeup air valve 72 isconnected to a pressurized air source 73, depicted schematically inFIG. 1. As shown at the top of FIG. 1, the second powder receiver 66 andfirst feed hopper 68 are located proximate to the powder spray booth 12,but the transfer line 64 interconnecting the primary hopper 60 andsecond powder receiver 66 may be several hundred feet in length. Thefeed hopper 68 is connected by a line 67 to a third vacuum pump 69housed within the powder kitchen 14, and carries a powder pump 74 (SeeFIG. 5) which is connected by a line 76 to a robot hopper 78. The robothopper 78, in turn, is connected by a line 79 to the spray gun(s) 42associated with robot 40.

The right hand portion of powder kitchen 14, as depicted in FIG. 1,contains similar structure to that described above in connection withprimary hopper 60. Instead of receiving virgin powder coating materialfrom container 54, this portion of the powder kitchen 14 is primarilysupplied with collected, oversprayed powder from the collection andrecovery system 16 of powder spray booth 12. In the presently preferredembodiment, the powder kitchen 14 houses a reclaim hopper 80 coupled toa third powder receiver unit 82 of the same type as receiver units 58and 66. The third powder receiver unit 82 is connected by a line 83 to athird vacuum pump 84 located within the powder kitchen 14, and is joinedby a reclaim suction line 86 to the powder collection and recoverysystem 16 as discussed below. A second transfer line 88, carrying a gatevalve 90 and makeup air valve 92 connected to air source 73,interconnects the reclaim hopper 80 with a fourth powder receiver unit94. This fourth powder receiver unit 94 is coupled to a second feedhopper 96 located proximate the powder spray booth 12. As schematicallydepicted in FIGS. 1 and 5, the second feed hopper 96 includes a positivepressure powder pump 98 which supplies powder material through a line100 to the spray guns 46 associated with overhead gun manipulator 44.The fourth powder receiver unit 94 is connected to a fourth vacuum pump102, located within the powder kitchen 14, by a line 104.

In the presently preferred embodiment, the primary hopper 60, first feedhopper 68, robot hopper 78, reclaim hopper 80 and second feed hopper 96are each carried by an individual load cell 106A–E, respectively, of thetype commercially available under Model Nos. FLB-3672-1K and H1242PS-C500 from the Hardy Instruments Company. The load cells 106A–E are“zeroed” or adjusted to reflect a zero weight when each of theirassociated hoppers are empty of powder coating material. As discussedbelow, each load cell 106A–E is operative to measure the weight orquantity of powder coating material deposited in their associatedhoppers and produce a signal representative of such weight reading.These signals are transmitted to a Programmable Logic Controller 108(PLC), preferably of the type commercially available from Allen Bradleyof Cleveland, Ohio, under Model No. PLC-5. The controller 108, in turn,operates each of the vacuum pumps 62, 71, 84 and 102, as well as valves70, 72, 90 and 92, in response to the signals from load cells 106A–E.

Operation of Powder Coating System Depicted in FIG. 1.

A detailed discussion of the structure and operation of each individualelement of the powder coating system 10 is given below, but the overalloperation of one embodiment thereof can be described with reference tothe schematic representation of FIG. 1. Unlike many prior systems, thepowder coating system of this invention employs negative pressure totransmit the powder coating material from the powder kitchen 14 to thepowder spray booth 12. Additionally, the supply and transfer of powderis accomplished essentially automatically as the powder coatingoperation proceeds.

Referring initially to the left hand portion of the powder kitchen 14,virgin powder coating material is transferred from the source 54 whenthe controller 108 activates the first vacuum pump 62. The first vacuumpump 62 creates a negative pressure within the first powder receiver 58which, in turn, draws the virgin powder coating material from source 54through line 56 into the first powder receiver 58. As described below,the first powder receiver 58 discharges powder coating material into theprimary hopper 60, and the quantity of such powder coating materialreceived is monitored by the load cell 106A associated with primaryhopper 60. When a predetermined level or quantity of powder coatingmaterial is present within primary hopper 60, its load cell 106A sends asignal representative of this condition to the controller 108, which, inturn, deactivates the first vacuum pump 62.

The transfer of powder coating material from primary hopper 60 to thefirst feed hopper 68 is also accomplished under the application ofnegative pressure. The controller 108 activates the second vacuum pump69 to create a negative pressure within the second powder receiver 66associated with first feed hopper 68. This negative pressure drawspowder coating material from the primary hopper 60 into transfer line64, and through the gate valve 70 therein which is opened by controller108 simultaneously with the activation of second vacuum pump 69. Thetransfer of powder from primary hopper 60 is monitored by its load cell106A which sends a signal to controller 108 when a predeterminedquantity or weight of powder is emitted from primary hopper 60. Thecontroller 108, in turn, closes the gate valve 70 within, transfer line64 to stop the flow of powder therethrough and turns off the secondvacuum pump 69. Filling of the first feed hopper 68 with powder from theprimary hopper 60 is accomplished by monitoring the weight or quantityof powder therein by its associated load cell 106B. When the quantity ofpowder in first feed hopper 68 falls below a predetermined level, itsload cell 106B sends a signal to controller 108 to activate a meteringdevice contained within the second powder receiver 66, as discussed indetail below. The powder transferred from primary hopper 60 to thesecond powder receiver 66 is then directed into the first feed hopper 68until a predetermined weight is obtained therein, at which time a signalfrom load cell 106B to controller 108 causes the metering device withinsecond powder receiver 66 to cease operation.

As schematically depicted at the top of FIG. 1, the powder coatingmaterial within the first feed hopper 68 is removed by the powder pump74 (see also FIG. 5), under the application of positive pressure, andtransmitted via line 76 into the robot hopper 78 carried by its ownload, cell 106C Once the robot hopper 78 receives a sufficient quantityof powder coating material, as monitored by load cell 106C, the powderpump 74 is deactivated by controller 108 and a second powder pump 77transfers the powder coating material from robot hopper 78 via line 79to the spray guns 42 associated with robot 40 for application onto thevehicle body 32.

The purpose of the load cells 106A–E is to provide for essentiallyautomatic operation of the system 10 so that the flow rate and totalquantity of powder coating material being transferred through the systemkeeps pace with the demand as a given number of vehicle bodies 32 passthrough the powder spray booth 12. The load cells 106A–C associated withprimary hopper 60, first feed hopper 68 and robot hopper 78,respectively, are each operative to monitor the quantity or weight ofpowder coating material therein and provide a signal to the controller108 in the event the quantity of powder falls below a predeterminedlevel. When the controller 108 receives such signals, the appropriatevacuum pump or metering device is activated to transfer powder coatingmaterial into the hopper(s) whose supply of coating material has beendepleted. In this manner, all of the hoppers 60, 68 and 78 have acontinuous, adequate supply of powder coating material.

Because of the extreme length of transfer line 64, the powder kitchen 14includes a valving arrangement to avoid, the presence of residual powdercoating material within transfer line 64 when the second vacuum pump 69is turned off to stop the flow of powder coating material from theprimary hopper 60 to the second powder receiver 66. As noted above,during the transfer operation from primary hopper 60 through secondpowder receiver 66, the controller 108 opens gate valve 70 withintransfer line 64. When the load cell 106A associated with primary hopper60 indicates a predetermined quantity of powder has been emittedtherefrom, the controller 108 deactivates second vacuum pump 69, closesgate valve 70 and opens makeup air valve 72 within the powder kitchen14. Pressurized air from the air source 73 then enters the transfer line64 through makeup air valve 72 to “chase” or positively force thecoating material which remains in transfer line 64 upstream from thepowder kitchen 14 into the second powder receiver 66. This substantiallyprevents any accumulation of powder coating material within the transferline 64 so that subsequent transfer operations of powder from theprimary hopper 60 to the first feed hopper 68 can be performed quicklyand efficiently.

With reference to the right hand portion of the powder kitchen 14, andtop right hand portion of FIG. 1, the components of the powder transfersystem which supply powder coating material to the spray guns 46 aredepicted. As discussed above, such elements include the reclaim hopper80, third powder receiver 82 and third and, fourth vacuum pumps 84, 102within the powder kitchen 14; and, the fourth powder receiver 94, secondfeed hopper 96 and third powder pump 98 located proximate the powderspray booth 12. The structure and operation of these elements isessentially identical to their counterparts on the left hand portion ofFIG. 1, except that instead of transmitting solely virgin powder coatingmaterial from the powder kitchen 14 to the spray booth 12 such elementstransmit primarily collected, oversprayed powder coating materialreceived from the collection and recovery system 16.

In order to fill the reclaim hopper 80 with oversprayed powder material,the third vacuum pump 84 is activated by controller 108 which creates anegative pressure within third powder receiver 82 to draw powder coatingmaterial via reclaim line 86 from the collection and recovery system 16into the third powder receiver 82. In a manner fully discussed below,the third powder receiver 82 deposits the oversprayed powder materialinto the reclaim hopper 80. The quantity of the powder entering thereclaim hopper 80 is monitored by load cell 106D associated therewith.From the reclaim hopper 80, the powder material is transferred to thefourth powder receiver 94 and second feed hopper 96 when the controller108 activates fourth vacuum pump 102. The negative pressure createdwithin the fourth powder receiver 94 pulls powder from the reclaimhopper 80 into second transfer line 88, through the gate valve 90 openedby controller 108, and into the interior of fourth powder receiver 94.The second feed hopper 96 receives such powder from the fourth powderreceiver 94, the quantity of which is monitored by load cell 106Eassociated therewith, and the positive pressure powder pump 98subsequently transfers the powder from second feed hopper 96 throughline 100 to the spray guns 46 carried by gun manipulator 44. Theoperation of vacuum pumps 84 and 102, and the metering device associatedwith fourth powder receiver 94, is governed, by the controller 108 inthe same manner as discussed above, i.e. in response to signals from theload cells 106D and 106E associated with the reclaim hopper 80 andsecond feed hopper 96, respectively. The operation of the positivepressure powder pump 98 is also governed by controller 108 dependingupon the presence of vehicle bodies 32 within the powder spray booth 12.Valves 90 and 92 within the powder kitchen 14 function in the identicalmanner as valves 70 and 72 described above.

Before discussing each of the individual elements associated with thepowder transfer system in detail, two additional features of the powdertransfer system should be noted. It is contemplated that in someapplications the total quantity of powder coating material required fromthe reclaim hopper 80 may exceed the amount of oversprayed, powdercoating material supplied thereto by the collection and recovery system16. In order to ensure that a sufficient quantity of powder coatingmaterial is always present within reclaim hopper 80, the primary hopper60 containing virgin powder coating material includes a powder pump 110connected by a line 112 to a minicyclone 114 carried by the reclaimhopper 80. This minicyclone 114 is commercially available from NordsonCorporation of Amherst, Ohio under Model No. PC-4-2. In the event theload cell 106D associated with reclaim hopper 80 senses less than therequired weight of powder material within reclaim hopper 80, andsufficient powder cannot be supplied from the collection and recoverysystem 16, then the controller 108 activates powder pump 110 to transfervirgin powder coating material through line 112 and minicyclone 114 intothe reclaim hopper 80 to supplement the total amount of powder therein.If such transfer is required, both virgin powder coating material andoversprayed, collected powder coating material from the booth 12 areintermixed within the reclaim hopper 80 and subsequently supplied to thespray guns 46 in the manner described above.

One further aspect of the powder transfer system shown in FIG. 1involves the utilization of a vent utility collector 116 located withinthe powder kitchen 14 which is connected by a line 118 to a vent 120 atthe top of primary hopper 60. Similarly, a second vent utility collector122, also contained within the powder kitchen 14, is connected by a line124 to the vent 126 of reclaim hopper 80. Each of the vent utilitycollectors 116, 122 is operative to provide ventilation to the interiorof the primary and reclaim hoppers 60, 82, respectively, and to remove“fines” from the upper portion of the interior of such hoppers 60, 82.The term “fines” as used herein refers to very small diameter particlesof powder material, under 10μ which usually concentrate near the upperportion of powder supply hoppers and are so small that they often do notbecome electrostatically charged when emitted from spray guns such asspray guns 42 and 46, nor do they have sufficient momentum to reach thearticle to be coated. Such small particles are usually not attracted tothe surface of an article to be coated and therefore tend to collectwithin the system which reduces transfer efficiency, i.e. the proportionof particles which adhere to an article to be coated. These smallparticles or fines are therefore advantageously removed by the ventutility collectors 116 and 122 within the powder kitchen 14 forsubsequent disposal.

Powder Coating System of FIG. 10 and Method of Operation.

With reference to FIG. 10, an alternative embodiment of a powder coatingsystem 500 according to this invention is schematically depicted. Thepowder coating system 500 contains a number of the elements shown inFIG. 1 and described above, and therefore the same reference numbers areutilized in FIG. 10 to identify structure which is common to FIG. 1.

The principal distinction between the system 500 of FIG. 10 and thesystem 10 of FIG. 1 is based upon the recognition that for certain sprayapplications with various types of virgin powder coating materials, caremust be taken to avoid excessive buildup of “fines.” The term “fines”refers to powder particles having a size of less than about 10 microns.As noted above, excessive buildup of fines has been found to createproblems of poor fluidization within the hoppers 60, 68, impact fusionon the parts to be coated, blinding of filter cartridges and sievescreens, increased powder buildup on the spray booth 12 and variouscoating dispensers 42, 46 and poor transfer efficiency. For many typesof powder coating materials, a buildup of fines greater than or equal toabout 30% of the total volume of the powder coating material results inone or more of the above named problems, although different percentagesof fines can present problems in the application of other types ofpowder coating materials.

In addition to concerns over the buildup of excessive volumes of fineswithin the powder coating material supplied to spray guns 42, 46,appearance of the finished product is also a factor to be consideredwhen applying particulate powdered coating material. For example, as thepercentage of larger or courser particles increases, the surface finishtends to degrade e.g. particles greater than about 70 microns fail toflow out on the surface of the part being coated to the same extent asparticles of about 10 microns, thus resulting in a rough surface finish.On the other hand, while smaller particles result in a better surfacefinish, the problems noted above are prevalent when the relativepercentage of such small particles exceeds a predetermined level.

In order to address the problems of controlling excessive fine buildupwhile retaining acceptable surface finish, the system 500 shown in FIG.10 has a powder kitchen 14 containing a mixing hopper 502 which isconnected by a pair of supply lines 504, 505 to the primary hopper 60containing virgin particulate powder material, and by a pair of supplyline 506, 507 to the reclaim hopper 80 which receives oversprayed powderfrom the booth 12 as described above. A gate valve 508 is positioned ineach of the supply lines 504, 505, respectively, extending from primaryhopper 60 into mixing hopper 502, and lines 506, 507 between reclaimhopper 80 and mixing hopper 502 each mount a gate valve 509. The mixinghopper 502, in turn, is connected by a transfer line 512 to the fourthpowder receiver unit 94, which, in the embodiment of FIG. 1, had beenconnected by line 88 to the reclaim hopper 80.

In addition to the foregoing, other structural changes in system 500compared to the coating system 10 shown in FIG. 1, include the use of asingle vacuum pump 514 connected by line 516 to each of the powderreceivers 66 and 94. As described above, two vacuum pumps, 69 and 102,were utilized in the embodiment of FIG. 1 to provide a negative pressurefor the transfer of coating material into the powder receiver 66, 94,respectively. Additionally, a single vent utility connector 518 isemployed in the embodiment of FIG. 10 instead of the two vent utilitycollectors 116, 122 depicted in FIG. 1. Vent utility collector 518,which has a greater capacity than those described above, is connected bya line 520 to a filter unit 522 as shown at the righthand portion of thepowder kitchen 14 of FIG. 10. The filter unit 522, in turn, is connectedby a duct 524 to a fan 526. The vent utility collector 518 is connectedby a line 519 to primary hopper 60, by a line 521 to reclaim hopper 80and by a line 523 to mixing hopper 502.

As noted above in connection with a discussion of the vent utilitycollectors 116, 122 of FIG. 1, the purpose of vent utility collector 518is to remove at least some of the fines present within the primaryhopper 60, reclaim hopper 80 and mixing hopper 502 to avoid excessivefine buildup and the attendant problems with same described above.Nevertheless, the vent utility collectors 116, 122 and/or 518 are not bythemselves sufficient to properly control the relative volume percentageof fines within the overall mixture of the virgin powder coatingmaterial and reclaim powder coating material supplied to at least someof the spray guns.

The powder coating system 500 of FIG. 10 operates in the same mannerdescribed above in connection with FIG. 1 except for the supply of themixture of virgin powder coating material and reclaimed or oversprayedpowder coating material supplied to coating dispensers 46. Instead ofsupplying the overspray powder material from reclaim hopper 80 directlyto guns 46 as shown in FIG. 1 and noted above, the system 500 of thisinvention provides a means and method of operation to intermix virginpowder coating material with oversprayed or reclaim powder coatingmaterial in appropriate volume percentages to insure that an excessivefine buildup does not occur within the mixing hopper 502, and that anappropriate particle size distribution is obtained in such mixturesupplied to the coating dispensers 46. The gate valves 508 associatedwith the virgin powder supply line 504, 505 and the gate valves 509associated with the reclaim powder supply lines 506, 507, are operatedby the controller 108 in accordance with a mathematical model executedby software within the controller 108. The purpose of the mathematicalmodel is to predict the particle size distribution within the mixinghopper 502 at steady state operation, which can then be used todetermine how much virgin powder coating material from feed hopper 60must be added to the mixing hopper 502 for combination with the reclaimor oversprayed powder supplied to hopper 502 from reclaim hopper 80 toobtain “stable” operation i.e. an absence of excessive fines and aresultant powder mixture which can be readily fluidized, pumped andsprayed onto parts to be coated.

Referring initially to FIGS. 12 and 13, graphical depictions areprovided of the particle size distribution within powder coatingmaterials. The term “particle size distribution” refers to the volumepercentage of powder particles within a particular size range in a givensample of powder coating material. FIG. 12 depicts the particle sizedistribution, of virgin powder coating material sold under No.158E114manufactured by Ferro of Cleveland, Ohio. This virgin powdercoating material has a median particle size of 22 microns, and the datapoints shown in the graph represent the volume percent of a total ofsixteen particle size ranges within the virgin powder coating materialof this type, as physically measured by a laser diffraction particlesize analyzer such as a Malvern PSD analyzer commercially available fromMalvern Instruments, Inc. of Southborough, Mass. The sixteen particlesize ranges, e.g. 0.5–1.9μ, 1.9–2.4μ, etc., were chosen for ease ofillustration, and it is contemplated that various other particle sizeranges could, be employed in the following description of the method ofthis invention.

FIG. 13 is a graphical depiction similar to FIG. 12, except it isrepresentative of the particle size distribution within particulatepowder coating material which has been reclaimed or collected from thepowder spray booth 12 from virgin powder coating material applied in onecoating application onto a given vehicle body 32 within booth 12. Inother words, FIG. 13 depicts the particle size distribution of powdercoating material which did not adhere to a vehicle body 32 within spraybooth 12 after a single spray operation with the virgin powder coatingmaterial depicted in FIG. 12. The particle size distribution of thepowder shown in FIG. 13 was also measured by the Malvern PSD analyzerand, as shown, a greater percentage of smaller particles are present inthe one pass oversprayed powder sample of FIG. 13 than in the virginpowder coating material of FIG. 12. This is generally due to the factthat larger powder particles are more easily and efficientlyelectrostatically charged by the coating dispensers 42 or 46 prior todeposition onto an object, and the larger mass of such larger particlesprovides them with greater momentum to flow to the vehicle body 32within the spray booth 12.

The purpose of the mathematical model of this invention is tomathematically predict the particle size distribution of the powderwhich remains in the system, i.e. which does not adhere to a vehiclebody 32 within booth 12, so that sufficient virgin powder coatingmaterial from primary hopper 60 can be introduced into mixing hopper 502on a continuous basis during steady state operation of system 500 toavoid the buildup of excessive fines and maintain an acceptable overallparticle size distribution.

In order to calculate the change in particle size distribution overtime, a determination must be made of the probability of a particleremaining in the system. A brief description is provided below of howprobability factors for each size range of the distribution aredetermined, followed, by the mathematical details. The objective here isto find a set of numbers (probability factors) which, when multiplied bythe virgin particle size distribution, will result in the oversprayedparticle size distribution.

-   -   (i) Begin by obtaining a ratio of the virgin particle size        distribution to the reclaim particle size distribution for each        of the size ranges. This ratio is a measure of the relative        tendency of the various particle sizes to be attracted to the        part. Particle sizes having a ratio of one or greater will more        likely be attracted to the part than particle sizes having a        ratio of less than one. These latter particles will have a        tendency to remain in the system.    -   (ii) Next, normalize the cumulative sum of the data from        Paragraph (i) to 1.00 to ensure that we continue to have a        probability distribution which accounts for 100% of all the        particles.    -   (iii) The probability of powder remaining in the system is        calculated by subtracting the above probability distribution for        each particle size from the value “one” and normalizing.        However, this mathematical operation still does not provide the        required probability distribution. What we are looking for is a        set of numbers which, when multiplied by the virgin particle        size distribution, will, result in the oversprayed particle size        distribution.    -   (iv) An appropriate multiplier factor for each particle size is        used on the distribution in Paragraph (iii) above in order to        get a good match between this calculated particle size        distribution and the particle size distribution of the        oversprayed powder. This new set of numbers is also normalized,        and the resulting values are the probability factors which are        multiplied times the particle size distribution for each cycle        through the system.

The initial step in the method outlined above involves physicallymeasuring the particle size distribution, F_(v), of the virgin powdercoating material to be used in a given coating application using theMalvern PSD analyzer mentioned above. This material is then sprayed intoa spray booth 12 of given configuration, with a particular vehicle body32 to be coated present, and the oversprayed or reclaim powder coatingmaterial is then collected. For purposes of the present discussion, suchoversprayed powder material is referred to as “one-pass reclaim,” i.e.the powder reclaimed after one “pass” or spraying operation. Theparticle size distribution of the one-pass reclaim is then physicallymeasured to obtain F_(r).

As noted in FIGS. 12 and 13, the particle size “distribution” refers tothe volume percentage of a total of sixteen (16) discrete particle sizeranges from 0.5 microns to 188 microns. The terms F_(v) and F_(r) aretherefore expressed as follows:

$\begin{matrix}\begin{matrix}{F_{v} = F_{v_{1 - 16}}} \\{= {F_{v_{1}}F_{v_{2}}\mspace{14mu}\ldots\mspace{14mu} F_{v_{1 - 16}}}}\end{matrix} & (1)\end{matrix}$where F_(v) ₁ =volume percentage of particles of virgin powder coatingmaterial having a size between 0.5–1.9μ

-   F_(v) ₂ =volume percentage of particles of virgin powder coating    material having a size between 1.9–2.4μ    -   etc.

$\begin{matrix}\begin{matrix}{F_{r} = F_{r_{1 - 16}}} \\{{= F_{r_{1}}},F_{r_{2}},{\ldots\mspace{11mu} F_{r_{1 - 16}}}}\end{matrix} & (2)\end{matrix}$where F_(r) ₁ =volume percentage of particles of one-pass reclaim powdercoating material having a size between 0.5–1.9μ

-   F_(r) ₂ =volume percentage of particles of one-pass reclaim powder    coating material having a size between 1.9–2.4μ    -   etc.

The measured particle size distributions F_(v) and F_(r) are input tothe controller 108 which is effective to calculate the quotient of theparticle size distribution as follows:

$\begin{matrix}\begin{matrix}{\frac{F_{v}}{F_{r}} = \frac{F_{v_{1 - 16}}}{F_{r_{1 - 16}}}} \\{{= \frac{F_{v_{1}}}{F_{r_{1}}}},{\frac{F_{v_{2}}}{F_{r_{2}}}\mspace{14mu}\ldots\mspace{20mu}\frac{F_{v_{16}}}{F_{r_{16}}}}}\end{matrix} & (3)\end{matrix}$

The quotients obtained from formula (3) noted above are then normalizedin accordance with the following relationship:

$\begin{matrix}{1 = {\sum\frac{F_{v}}{F_{r}}}} & (4)\end{matrix}$

This is accomplished in a two-step process wherein initially the sum ofthe quotients of F_(v)/F_(r) or S, is obtained:

$\begin{matrix}{S = {\frac{F_{v_{1}}}{F_{r_{1}}} + {\frac{F_{v_{2}}}{F_{r_{2}}}\mspace{14mu}\ldots\mspace{14mu}\frac{F_{v_{16}}}{F_{r_{16}}}}}} & (5)\end{matrix}$

Thereafter, the “normalized” value of the various quotients for each ofthe sixteen size ranges of particles shown in the graph of FIGS. 12 and13 are calculated as follows:

$\begin{matrix}{1 = {\frac{F_{v_{1}}/F_{r_{1}}}{S} + {\frac{F_{v_{2}}/F_{r_{2}}}{S}\mspace{14mu}\cdots\mspace{14mu}\frac{F_{v_{16}}/F_{r_{16}}}{S}}}} & (6)\end{matrix}$

The next sequence of calculations executed by software within thecontroller 108 leads to a solution for probability factors P, e.g.P₁₋₁₆, for each particle size range. The probability factors P₁₋₁₆ arerepresentative of the probability that particles within each of thesixteen size ranges depicted in FIGS. 12 and 13 will remain in thesystem 500, i.e. will not be attracted to the vehicle body 32 withinbooth 12 and therefore are reclaimed as oversprayed powder. Thisinformation is important because, as noted above, system stability isdependent on maintaining the volume percentage of fines within mixinghopper 502 below a predetermined level, such as 30%. The formulautilized to obtain the probability factor P₁₋₁₆ is given as follows:

$\begin{matrix}{P = {{F_{A}\left( {1 - {{normalized}{\sum\limits_{x}{F_{v}F_{r}}}}} \right)}x}} & (7)\end{matrix}$where: F_(A)=adjustment factor

-   -   X=integers 1–16    -   P=probability factors P₁, P₂ . . . P₁₆        The portion of equation (7) after the F_(A) term is determined        by the following two step calculation. First, a factor Z is        calculated as follows:

$\begin{matrix}\begin{matrix}{Z = {\sum\limits_{x}{\left( {1 - \frac{F_{v}/F_{r}}{S}} \right)x}}} \\{= {\left( {1 - \frac{F_{v_{1}}/F_{r_{1}}}{S}} \right) + {\left( {1 - \frac{F_{v_{2}}/F_{r_{2}}}{S}} \right)\mspace{14mu}\cdots\mspace{14mu}\left( {1 - \frac{F_{v_{16}}/F_{r_{16}}}{S}} \right)}}}\end{matrix} & (8)\end{matrix}$The quotients obtained from the calculations based on equation (8) arethen normalized to one (1):

$\begin{matrix}{1 = {{\left( {1 - \frac{F_{v_{1}}/F_{r_{1}}}{S}} \right) \div Z} + {\left( {1 - \frac{F_{v_{2}}/F_{r_{2}}}{S}} \right) \div Z} + {\cdots\mspace{14mu}{\left( {1 - \frac{F_{v_{16}}/F_{r_{16}}}{S}} \right) \div Z}}}} & (9)\end{matrix}$Each of the quotients obtained from the calculations based on equation(9) represents a normalized value for each of the sixteen particle sizeranges from the graphs in FIGS. 12 and 13. These values are then used inequation (7) to obtain probability factors P for each particle sizerange, i.e. a representation of the probability that the particleswithin each size range will remain in the system (not attach to a part)after a particular coating operation.

For example, equation (7) can be written to obtain the probabilityfactor P₁ for the powder particles having a size in the first group ofparticles, 0.5–1.9 microns, as follows:

$\begin{matrix}{P_{1} = {F_{A_{1}}\left( {1 - {{normalized}\mspace{14mu}\frac{F_{v_{1}}}{F_{r_{1}}}}} \right)}} & (10)\end{matrix}$The “adjustment factor” F_(A) is obtained empirically by trial and errorfor each of the sixteen groups of particle sizes, such that thefollowing equations are accurate compared to actual measurements ofvirgin and one-pass reclaim particles:F _(r) ₁ =F _(v) ₁ ·P ₁  (11)F _(r) ₂ =F _(v) ₂ ·P ₂  (12)F _(r) ₁₆ =F _(v) ₁₆ ·P ₁₆  (13)That is, the actual measurements of particle size distribution F_(v) andF_(r) obtained initially by the laser diffraction particle sizeanalyzer, as noted in equations (1) and (2), are employed to derivesuitable adjustment values F_(A) to ensure that the ultimate mathematiccalculations of F_(r) ₁₆ are as accurate as possible. Given the type ofpowder coating material noted above, the actual adjustment factors F_(A)₁₋₁₆ for the particle size ranges 0.5–1.9μ increasing to 87.2–188μ arepreferably 1.0, 0.95, 0.90 . . . , 0.25, respectively.

Once each of the probability factors P₁₋₁₆ are calculated as set forthabove, these values must also be normalized to 1.0 in the same manner asdescribed above. The resulting, normalized values for P₁₋₁₆ can then beutilized as multipliers, e.g. times the particle size distribution ofthe virgin powder, to provide a mathematical prediction of theoversprayed particle size distribution for each cycle through theapparatus 10.

The foregoing discussion was based upon a mathematical model in which itwas assumed, for simplicity, that no additional virgin powder materialis added to the system, and the original quantity of powder material isrecycled in successive coating operations. Because powder coatingmaterial adheres to objects such as vehicle bodies 32 within booth 12under normal operating conditions and must be continuously replaced, amathematical model which accounts for a given fraction of virgin powdercoating material added to the system is required to approximate theactual operation of system 500.

Without repeating the calculations given above in equations (1)–(13),the following relationship is employed to represent the particle sizedistribution of the one-pass reclaim from equations (11)–(13):F _(r) =F _(v) ·P  (14)The term F_(r) therefore represents the particle size distributionwithin the first-pass reclaim powder, as calculated using themathematical model of equations (1)–(13).

After normalizing equation (14) to 1.0, and accounting for the additionof a volume fraction percentage y of new virgin powder added to thereclaim powder after the first pass or coating operation, the followingrelationship is derived:F _(r) _(i) _(y)=(1−y)F _(r) _(i) +yF _(v)  (15)Where: y=volume fraction percentage of virgin powder coating materialadded to the system after the one-pass reclaim powder is collected.

-   -   F_(r) _(i) _(y)=particle size distribution of one-pass reclaim        containing a fraction y of virgin powder coating material.    -   i=the index for the number of passes beginning with virgin        powder coating material.

Having calculated a value for F_(riy), the particle distribution of asubsequent “pass” or coating operation can be expressed as follows:F _(r) _(i+1) =F _(r) _(i) _(y) ·P  (16)Equation (16) is normalized to 1.0, and then the following calculationis made:F _(r) _(i+1) _(y)=(1−y)·Fr _(i+1) +yF _(v)  (17)The term F_(r) _(i+1) _(y) represents the particle distribution withinthe second pass reclaim powder (i+1) containing a volume fraction y ofvirgin powder.

This same series of calculations is repeated for a number of cycles sothat curves of the type depicted in the graph of FIG. 14 can begenerated. FIG. 14 is a representation of the volume percentage of finespresent when spraying Ferro 158E 114 powder coating material wherein thefraction or volume percentage of virgin powder is varied. Each curve onthe graph of FIG. 14 represents a different volume percentage of virginpowder y within the reclaim, and depicts how the volume percentage offines changes with the number of cycles or successive coatingoperations. Based on calculations from equations (15) and (17) andsuccessive iterations of same for all particle sizes (i+16), acalculated, particle size distribution over the entire range of thesixteen particle size ranges is obtained. Software within the controller108 is operative to sum the calculated volume percentage of allparticles less than or equal to ten (10) microns within each of thesixteen size ranges, to obtain the values on the ordinate of the graphat FIG. 14 for each successive cycle or coating operation. Calculationsfor a number of different y values are included in FIG. 14 for purposesof illustration to provide an indication of how the volume percentage offines varies with successive cycles and with differing fractions ofvirgin powder added to the reclaim powder.

The curves of FIG. 14 were mathematically derived from the aboveequations using Ferro 158E114 virgin powder coating material having amedian particle size of about 22 microns. For this type of material, ithas been found that the volume percentage of fines should be maintainedless than about 30% to avoid the problems of excessive fines buildupnoted above. From the graph of FIG. 14 it is observed that in order tomaintain the volume percentage of fines less than 30%, a fraction y ofvirgin powder greater than 65% or on the, order of about 70% should beadded to the system 500 during steady state operation. This isaccomplished by operation of the controller 108 which selectively opensand closes the gate valve 508 in each line 504, 505 from the primaryhopper 60 to the mixing hopper 502, and the gate valve 509 in each line506, 507 extending between the reclaim and mixing hoppers 80, 502.

As described above, the mathematical model employed herein is useful toprovide an indication of the percentage of fines within the mixinghopper 502 for powder coating materials containing different volumepercentages of virgin powder. In turn, the appropriate volume fractionof virgin powder can be added to the mixing hopper 502 during steadystate operation of the system 500 to avoid the accumulation of excessivefines within mixing hopper 502. As a safety precaution, the system 500of this invention also includes software contained within the controller108 which monitors the particle size distribution within the mixinghopper 502 to ensure that the actual percentage of fines within themixing chamber 502 is consistent with that predicted by the mathematicalmodel.

With reference to FIG. 15, a flow chart is provided which,diagrammatically depicts the sequence of operations to perform theabove-described monitoring function. Initially, a sample is manuallywithdrawn from the mixing hopper 502 and the same laser diffractionparticle size analyzer mentioned above is employed to measure the actualparticle size distribution within mixing hopper 502, F_(M), asschematically depicted by block 528. The particle size distributioninformation is input from block 528 either manually or electronically tothe controller 108 via line 530. Thereafter, all operations depicted inFIG. 15 are executed electronically within the software of controller108. The controller 108 operates to calculate the volume percentage offines, e.g. particles less than about 10 microns, contained within themixing hopper 502. See block 532. A signal representative of suchpercentage volume calculation is input by line 534 to block 536 where acomparison is made between the calculated volume percentage and apredetermined maximum volume percentage of fines for the particular typeof powder coating material being applied by system 500. In theillustrated embodiment shown in FIG. 15, a desired maximum volumepercentage of 30% is nominally shown in block 536, although it should beunderstood that other minimum volume percentages of fines may be moreappropriate for other types of coating materials. If the calculatedpercentage of fines is less than or equal to 30%, a “no” signal is sentby line 538 back to block 528, and the monitoring sequence is terminateduntil the next monitoring period begins, e.g. one day, one week or otherdesired periods.

In the event the calculated percentage of fines is determined withinblock 536 to exceed 30% by volume of the powder within mixing chamber502, a signal is sent from block 536 through line 540 to block 542. Thecalculation performed by the controller 108 as denoted in block 542involves the empirical selection of a weight fraction W, i.e. afractional percentage such as 20%, 30% etc. The controller 108 thensolves the following equation using the selected weight fraction W:F _(f) =W·F _(v)+(1−W)·F _(m)  (18)Where: F_(f)=desired volume percentage of fines within mixing hopper,e.g. <30%

-   -   W=weight fraction of virgin powder    -   F_(m)=particle size distribution of powder within mixing hopper    -   F_(v)=particle size distribution of virgin powder        In the event the weight fraction W selected yields an F_(f)        value greater than 30%, then another, higher weight fraction W        is selected and the calculations using equation (18) are        repeated.

The next step in the monitoring sequence depicted in FIG. 15 is to weighthe powder within the mixing hopper, 502 using a load cell 106 of thetype described above in connection with FIG. 1. See Box 544. The actualweight of the powder within mixing hopper 502 is then compared with thetotal weight capacity of mixing hopper 502, as schematically shown inblock 546, and a signal representative of such calculation is input vialine 548 to block 550. As depicted in block 550, the controller 102 isoperative to cause virgin powder coating material to enter into themixing hopper 502 in the event the level of powder within the mixinghopper 502 is less than or equal to 1−W (block 552). If the mixinghopper 502 is too full to receive the weight fraction W of virgin powdercoating material necessary to reduce the volume fraction of fines to thedesired level, i.e. >(1−W), then the controller 108 initially causes themixing hopper 502 to dump sufficient powder therefrom as depicted inblock 554 before virgin powder coating material is added. The quantityof powder within the mixing hopper 502 which must be dumped or removedto make room for the virgin particulate powder coating material to beadded can be determined from the following relationship:

$\begin{matrix}{Q_{D} = {\left( {\frac{{MH}_{w}}{{MH}_{c}} + W} \right) - 1.0}} & (19)\end{matrix}$where: Q_(D)=fraction of powder to be dumped from mixing hopper

-   -   MH_(W)=mixing hopper weight, as measured    -   MH_(C)=mixing hopper capacity

The above-described series of weight calculations is therefore intendedto make sure that there is sufficient capacity within the mixing hopper502 to receive new virgin powder coating material, and therefore reducethe overall volume fraction of fines therein, without overflowing themixing hopper 502. After the virgin powder coating material is added(block 552), the monitoring operation is terminated until the nextmonitoring period.

The above-described method of maintaining the desired proportion ofreclaim powder coating material and virgin powder coating materialwithin mixing hopper 502 is therefore dependent upon measurements ofweight loss within the mixing hopper 502 as the coating operationproceeds. Alternatively, it is contemplated that the mixing hopper 502could be supplied with the appropriate quantities of reclaim and virginpowder coating material based upon flow rate measurements instead ofweight measurements. In this embodiment, the flow rate of powder coatingmaterial discharged from mixing hopper 502 is monitored over time, andpredetermined quantities of both reclaim powder coating material andvirgin powder coating material are added from the reclaim hopper 80 andprimary hopper 60, respectively, into the mixing hopper 502 using flowcontrol devices such as screw feeders (not shown). Preferably, a screwfeeder or similar device associated with the primary hopper 60, and aseparate screw feeder associated with the reclaim hopper 80, areactivated by controller 108 in response to a signal from mixing hopper502 and/or after a predetermined period of operation to introduceadditional virgin powder coating material and reclaim powder coatingmaterial into the mixing hopper 502.

Powder Receivers

Referring to FIG. 2, the powder receiver 58 mentioned above inconnection with a discussion of the system 10 of FIG. 1 is illustratedin detail. It should be understood that each of the other powderreceivers 66, 82 and 94 are structurally and functionally identical topowder receiver 58, and therefore only one of the powder receivers isdiscussed in detail herein. Additionally, an alternative embodiment of apowder receiver 600 is disclosed below with reference to FIG. 11.

The powder receiver 58 includes a collector housing 128 having a hollowinterior 130 within which a cartridge filter 132 is mounted by a plate134. An access panel 136 is releasably secured by latches 138 along oneside of the collector housing 128 to permit access to the cartridgefilter 132. The interior 130 of collector housing 128 is vented by avent 140, and its upper end is closed by a cap 142 secured thereto bylatches 144. The cap 142 mounts a reverse air jet valve 146 in alignmentwith the open end of cartridge filter 132 connected to plate 134. Thereverse air jet valve 146 is connected by a line 148 to an accumulator150 which, in turn, is connected to the source 73 of pressurized airdepicted schematically in FIG. 2. The cap 142 also carries a fitting 154connected to a suction hose or line 61 from the first vacuum pump 62.The lower portion of collector housing 128 includes a powder inlet 158connected to the line 56 from the container 54 carrying virgin powdercoating material. The collector housing 128 tapers radially inwardlyfrom the powder inlet 158, in a downward direction as depicted in FIG.2, forming a tapered base portion 160 which includes external flanges162.

As discussed above, in order for the load cell 106A associated withprimary hopper 60 to function properly it must be “zeroed” or set at azero weight reading with the primary hopper 60 completely empty ofpowder coating material. In this manner, only the powder coatingmaterial which actually enters the primary hopper 60 is weighed by theload cell 106A. In order to ensure an accurate weight reading of thepowder is obtained within primary hopper 60, all of the elementsassociated with the first powder receiver unit 58 are supportedindependently of the primary hopper 60 upon a frame 164 depicted in FIG.2. This frame 164 includes a top plate 166 supported on vertical legs168, angled braces 170 extending between the top plate 166 and verticallegs 168, and, one or more horizontal supports 172 located atintermediate positions in between the vertical legs 168.

The collector housing 128 is mounted to the top plate 166 of frame 164by bolts 174 extending between the external flange 162 of collectorhousing 128 and the top plate 166. Extending downwardly from the taperedbase portion 160 of collector housing 128 is a flexible sleeve 176 whichcouples the collector housing 128 with a rotary air lock metering device178 of the type commercially available from Premier Pneumatics, Inc. ofSalina, Kans. under Model No. MDR-F-G-76-10NH-2-RT-CHE-T3. The meteringdevice 178 is drivingly connected by a belt (not shown) to the output ofa motor 182 carried on a support plate 184 connected to one of thevertical legs 168. The motor 182 is operative to rotate a series ofinternal vanes 186 within the metering device 178 which transfer ametered quantity of powder coating material from the tapered baseportion 160 of collector housing 128 into a rotary sieve 196 mounted ona horizontal support 172. The rotary sieve 196 is a commerciallyavailable item of the type manufactured and sold by Azo Incorporated ofGermany under Model No. E-240. The rotary sieve 196, in turn, transfersthe powder coating material through a second flexible sleeve 198 intothe powder inlet 200 of primary hopper 60 which is shown in more detailin FIG. 3 and described below.

In operation, the first vacuum pump 62 is activated by controller 108drawing a vacuum along suction hose or line 61 to create a negativepressure within the hollow interior 130 of collector housing 128. Inturn, virgin powder coating material is drawn from the supply container54 through line 56 and powder inlet 158 into the hollow interior 130 ofcollector housing 128. Some of the powder coating material falls bygravity into the tapered base portion 160 of collector housing 128, andanother portion of the powder coating material collects on the walls ofthe cartridge filter 132. Periodically, pressurized air supplied fromthe accumulator 150 is transmitted in pulses through the reverse air jetvalve 146 aligned with cartridge filter 132. These jets of air dislodgethe powder coating material collected on the walls of filter 132allowing it to fall downwardly into the tapered base portion 160 ofcollector housing 128.

The powder coating material is transferred from the collector housing128 by the air lock metering device 178, in response to operation ofmotor 182, such that a metered quantity of powder coating materialenters the rotary sieve 196. After passing through the rotary sieve 196,the powder coating material falls by gravity through the flexible sleeve198 and into the powder inlet 200 of the primary hopper 60. When apredetermined quantity of powder coating material is collected withinprimary hopper 60, the load cell 106A associated therewith sends asignal to the controller 108, which, in turn, discontinues operation ofthe first vacuum pump 62. As mentioned above, all of the other powderreceiver units 66, 82 and 94 in the powder transfer system of FIG. 1 arestructurally and functionally identical.

With reference to FIG. 11, an alternative embodiment of a powderreceiver 600 is illustrated in detail. Powder receiver 600 is similar,in part, to powder receivers 58, 66, 82 and 94 described above inconnection with a discussion of FIG. 2, and the same reference numbersare used in FIG. 11 to identify structure common to that of FIG. 2. Onedifference between powder receiver 600 and powder receiver 58 is thestructure for transferring the powder coating material from thecollector housing 128 to the primary hopper 60. It has been found thatin some applications with certain types of powder material, intermittentflow stoppage has occurred due to arching or bridging of the coatingmaterial in the area of the tapered base portion 160 of powder receiver58. When the tapered base portion 160 becomes blocked, powder coatingmaterial cannot be transported through the airlock metering device 178into the rotary sieve 196 of the construction depicted in FIG. 2. Inaddition, the powder receiver 600 of FIG. 11 does not include a sieve196 but it is contemplated that one could be placed atop the hopper 60to sieve the particulate powder coating material prior to introductioninto the hopper 60. Preferably, the sieve 196 is utilized at least wherethe powder coating material is initially introduced into the system,e.g. at receivers 58 and 82. See FIG. 1.

The powder receiver 600 of FIG. 11 is essentially constructed of theupper portion of the powder receiver 58 of FIG. 2, and a lower portionincluding structure for fluidizing the powder coating material directedinto the collector housing 128 so that it can be smoothly transferred tothe primary hopper 60. The bottom portion of collector housing 128defines an interior including a fluidized bed 602 which extends betweenthe cap 142, and a porous plate 604 which extends outwardly from thesidewall of collector housing 128 and is supported thereto by brackets(not shown). A second area within the base portion of collector housing128 is an air plenum 608 which extends between the porous plate 604 anda circular mounting plate 610 carried by brackets mounted to thesidewall 129 of collector housing 128. A third area within the baseportion of the collector housing interior is a motor chamber 612extending between the mounting plate 610 and a bottom wall 614 ofcollector housing 128. The entire powder receiver 600 is preferablymounted atop a support stand 615 in position vertically above a primaryhopper 60 or other hopper, for purposes to become apparent below.

The base portion of feed hopper 600 is provided with an agitator 616which includes a motor 618 carried within the motor chamber 612 by amotor mount connected to the mounting plate 610. The output of motor 618is drivingly connected to a shaft 622 rotatably carried within a bearing624. The bearing 624 is mounted by a bearing mount to the mounting plate610 and extends vertically upwardly through the air plenum 608 to apoint immediately above the porous plate 604. At least two entrainmentarms 628 are secured by a lock nut 630 at the top of shaft 622 whichextends through bearing 624, so that in response to operation of motor618 the entrainment arms 628 are rotated with respect to the porousplate 604 at a location thereabove.

At least two air inlets 632 are connected by tubes to an air supply line636, in a manner not shown, which enters one side of the motor chamber612. This air supply line 636, in turn, is connected to the source ofpressurized air 73. An upwardly directed flow of air is provided throughthe air inlet 632 into the air plenum 608 where the air is deflected bybaffles 638 mounted to the bearing 624. The purpose of these baffles isfully disclosed in U.S. Pat. No. 5,018,909, owned by the assignee ofthis invention, the disclosure of which is incorporated by reference inits entirety herein.

A transfer tube 640 is connected at one end to the collector housing 128above the porous plate 604 and within the fluidized bed 602. The otherend of the transfer tube 640 mounts to the inlet 642 of primary hopper60. Preferably, a rotary air lock 178, driven by a motor 182, both ofthe same type described above in connection with a discussion of powderreceiver 58, is connected in the transfer tube 640. As schematicallydepicted in FIG. 11, operation of the motor 182, and, hence, the rotaryair lock 178, is controlled by the controller 108. In response to theoperation of rotary air lock 178, powder coating material from thefluidized bed 602 within powder receiver 600 flows by gravity downwardlythrough the transfer tube 640, and then into the interior of primaryhopper 60. The motor 182 is deactivated to stop operation of rotary airlock 178, as desired, to halt the flow of powder coating materialthrough transfer tube 640. It has been found that this configuration ofpowder receiver 600 provides a smooth transfer of powder to the primaryhopper 60, and it is contemplated that such powder receiver 600 could beutilized as an alternative in the embodiments of both FIGS. 1 and 10.The powder receiver 600 otherwise functions in the same manner as powderreceiver 58 described above, and the discussion of such operation is notrepeated herein.

Primary and Reclaim Hoppers

The primary hopper 60 and reclaim hopper 80 are essentially identical toone another, and, for purposes of discussion, only the primary hopper 60is illustrated and described in detail. With reference to FIGS. 3 and 4,the primary hopper 60 comprises a housing 202 having an internal wall204 in the general shape of a “figure 8”. As such, the internal wall 204includes two circular-shaped portions 206 and 208 which meet at areduced diameter area 210 at the center of housing 202 defined byopposed, triangular-shaped baffles 212 and 214 each connected to oneside of the housing 202. Each of the baffles 212, 214 have a pair ofside panels 216, 218 which extend inwardly from a wall of the housing202 and meet to form an apex 220 toward the center of the housinginterior 203.

As best shown in FIG. 4, a porous plate 222 is carried by mounts 224near the base of housing 202 which separates the housing interior 203into a fluidized bed 226 located between the porous plate 222 and thetop wall 228 of housing 202, and an air plenum 230 located between theporous plate 222 and the bottom wall 232 of the housing 202. The airplenum 230 contains a number of baffles 270 and a generally U-shaped,perforated air tube 272. The bottom wall 232 rests atop the load cell106A, discussed above in connection with the powder transfer system ofthis invention.

The top wall 228 of housing 202 supports a first agitator 234, a secondagitator 236 and an access cover 238 having a handle 240 and latchmechanisms 242 which is mounted by a hinge 243 over an opening 244 inthe top wall 228. This opening 244 is offset from the powder inlet 200of primary hopper 60 so that access to in housing interior 203 formaintenance or the like can be obtained without interference with thepowder inlet 200. The first agitator 234 includes a motor 246 connectedby a shaft 248 to a gear box 250. The output of gear box 250 isdrivingly connected to a shaft 252 encased within a tube 254. The lowerend of shaft 252 mounts at least two agitator paddles 256 which arerotatable within the circular portion 206 of the housing interior 203formed by internal wall 204, at a location vertically above the porousplate 222. The second agitator 236 has a similar construction to firstagitator 234. Second agitator 236 includes a motor 258 having a shaft260 connected to a gear box 262 whose output is drivingly connected to ashaft 264 encased within a tube 266. Two or more paddles 268 are mountedat the base of shaft 264 within the other circular portion 208 ofhousing interior 203 formed by internal wall 204. As depicted in FIG. 4,the shaft 264 and tube 266 associated with second agitator 236 areslightly longer than their counterparts in the first agitator 234 sothat the paddles 268 of second agitator 236 are located closer to theporous plate 222 than those of first agitator 234. The paddles 256, 268overlap but do not interfere with one another because of the verticaloffset.

As mentioned above, one aspect of this invention is to provide for thetransfer of large quantities of powder coating material e.g. on theorder of 300 pounds per hour and up, at flow rates of 1–2 pounds persecond, while maintaining the desired density and particle distributionwithin the flow of powder coating material. As noted above, the term“density” refers to the relative mixture or ratio of powder to air, andthe term “particle distribution” refers to the disbursion of powderparticles of different sizes within the flow of powder coating material.The primary hopper 60 and reclaim hopper 80 are designed to meet thedesired density and particle distribution requirements at highthroughputs of powder coating material.

In operation, pressurized air is introduced into the perforated air tube272 within air plenum 230 creating an upward flow of air which is evenlydistributed by the baffles 270 across the bottom of porous plate 222.Powder coating material is introduced into the housing interior 203through its powder inlet 200 and distributed along the porous plate 222by the upward, fluidizing air flow therethrough and by operation of thefirst and second agitators 234, 236. The “figure 8” shape of the housinginterior 203 defined by internal wall 204 substantially eliminates “deadspots” therein as the agitator paddles 256, 268 move relative to theporous plate 222 so that the powder coating material is evenlydistributed along the entire surface area of porous plate 222 andagglomeration or bunching up of the powder material is substantiallyeliminated. This produces an even, uniform powder distribution withinthe fluidized bed 226 having the desired particle distribution anddensity. In response to activation of the third vacuum pump 69, airentrained, powder coating material is withdrawn from the housing 202 ofprimary hopper 60 through a suction tube 274 inserted within the housinginterior 203, which, in turn, is connected to transfer line 64 describedabove.

Feed Hoppers

The first and second feed hoppers 68 and 96 are essentially identical inconstruction and therefore only the details of first feed hopper 68 arediscussed herein. With reference to FIG. 5, feed hopper 68 comprises ahousing 276 having a top wall 278 formed with an opening closed by acover 279, a substantially cylindrical-shaped side wall 280 and a bottomwall 282 carried by the load cell 106B. The housing 276 defines aninterior which is separated into essentially three discreet areas. Onearea is a fluidized bed 284 extending between the top wall 278 and aporous plate 286 which extends outwardly from the housing side wall 280and is supported thereto by brackets 288. A second area within thehousing 276 is air plenum 290 which extends between the porous plate 286and a circular mounting plate 292 carried by brackets 294 mounted to theside wall 280. The third area within the interior of housing 276 is amotor chamber 296 extending between the mounting plate 292 and bottomwall 282

The feed hopper 68 is provided with an agitator 298 which includes amotor 300 carried within the motor chamber 296 by a motor mount 302connected to the mounting plate 292. The output of motor 300 isdrivingly connected to a shaft 304 rotatably carried within a bearing306. The bearing 306 is mounted by a bearing mount 308 to the mountingplate 292 and extends vertically upwardly through the air plenum 290 toa point immediately above the porous plate 286. At least two paddles 308are secured by a lock nut 310 at the top of shaft 304 which extendsthrough bearing 306, so that in response to operation of motor 300 thepaddles 308 are rotated with respect to the porous plate 286 at alocation immediately thereabove.

At least two air inlets 312, carried by mounting plate 292, areconnected by tubes 314 to an air supply line 316, in a manner not shown,which enters one side of the motor chamber 296. This air supply line316, in turn, is connected to the source of pressurized air 73 describedabove in connection with the powder receivers. An upwardly directed flowof air is provided through the air inlets 312 into the air plenum 290where the air is deflected by baffles 318 mounted to the bearing 306.These baffles 318 are of the same type employed in powder receiver 600,and as disclosed in U.S. Pat. No. 5,018,909, mentioned above.

In operation, powder coating material is introduced into the fluidizedbed 284 of housing 276 through a tapered, powder inlet 320 mounted alongthe side wall 280 of housing 276. The motor 300 is operative to rotatepaddles 308 so that the powder coating material is evenly distributedalong the porous plate 286 with no dead spots. The powder coatingmaterial is fluidized along the porous plate 286 by the upwardlydirected flow of air from air supply line 316 and air inlets 312. Inorder to remove the powder coating material from housing 276, one ormore powder pumps such as pump 74 is operated to draw the powder coatingmaterial through a suction tube 322 which extends into the housinginterior immediately above the porous plate 286. A number of suctiontubes 322 are shown in FIG. 5 for purposes of illustrating that multiplepowder pumps 74 could be employed to draw powder from feed hopper 68.

Robot Hopper

The robot hopper 78 schematically depicted in FIG. 1 is shown in moredetail in FIG. 6. In the presently preferred embodiment, the robothopper 78 includes a cylindrical base forming a combined air plenum andmotor chamber 324 which houses a motor 326 drivingly connected to ashaft 328 whose upper end mounts one or more paddles 330. The topportion of robot hopper 78 includes a cylindrical housing 332 having atop wall 334 and a bottom wall formed by a porous plate 336 whichcommunicates with the air plenum and motor chamber 324. The cylindricalhousing 332 defines a fluidized bed 338 within which arectangular-shaped plate or baffle 340 is mounted. The baffle 340 isvertically spaced above the porous plate 336 and divides the fluidizedbed 338 into two sections. In one section or side of baffle 340, powdercoating material from feed hopper 68 is introduced through a powderinlet 342 schematically depicted at the top of the cylindrical housing332. A suction tube 344 associated with the powder pump 79 is mounted tocylindrical housing 332 on the-opposite side of baffle 340, and thissuction tube 344 terminates immediately above the porous plate 336.

The robot hopper 78 receives powder coating material via line 76 frompowder pump 74 associated with feed hopper 68. The powder coatingmaterial enters the powder inlet 342 of cylindrical housing 332 and isdirected downwardly along one side of baffle 340 onto the porous plate336. The motor 326 is operative to rotate paddles 330 immediately abovethe porous plate 336 so that a uniform flow of air entrained powdermaterial can be withdrawn by the powder pump 79 through suction tube 344for transmission to the robot 40 and its associated spray guns 42. Ithas been found that the presence of baffle 340 within the interior ofcylindrical housing 332 assists in stabilizing the fluidization ofpowder coating material across the porous plate 336 to ensure that thedesired density and powder distribution within the flow of powdercoating material withdrawn by powder pump 79 is maintained.

Powder Collection and Recovery System

With reference to FIGS. 1 and 7–9, the powder collection and recoverysystem 16 is illustrated in further detail. This system 16 is generallyrelated to that disclosed in U.S. Pat. No. 5,078,084 to Shutic, et al.,the disclosure of which is incorporated by reference in its entiretyherein. As noted above, the powder collection and recovery system 16 islocated below the floor 20 of powder spray booth 12 on either side ofthe center portion 36 of booth 12 along which the vehicle bodies 32 aretransported by conveyor 34. As depicted at the left hand portion of FIG.7, gratings 38 cover the booth floor 20 so that oversprayed, airentrained powder coating material can be drawn downwardly from any areawithin the booth interior 30 into the system 16.

The powder collection and recovery 16 is modular in construction andgenerally comprises a series of powder collection units 346 mountedside-by-side and extending longitudinally along the entire length of thebooth 12. See center of FIG. 7. The powder collection units 346 areconnected in groups of three or four, for example, to individual fan orblower units 348 located beneath the powder collection units 346, asshown, in FIG. 1 and the right side of FIG. 7. Each of the powdercollection units 346 includes a collector housing 350 having opposedside walls 354, 356, opposed end walls 358, 360 and an angled or slopedbottom wall 362. A, clean air chamber 364 is located at the top ofcollector housing 350 which is formed by a pair of inwardly angledsupport plates 366, 367 each having a number of spaced openings 368,opposed side plates 369, 370, and, a pair of access doors 371, 372 whichare hinged to the side plates 369, 370, respectively. The clean airchamber 364 extends across the length of collector housing 350 andconnects to an extension 373, the purpose of which is described below.The lower portion of collector housing 50 forms a powder collectionchamber 374 having tapered sidewalls and a bottom wall, defined by aporous plate 376. The porous plate 376 is mounted above the base 362 ofcollector housing 350, at an angle of approximately five degrees withrespect to horizontal, which forms an air plenum 377 therebetween. Anupwardly directive flow of air is introduced into the air plenum 377beneath the porous plate 376 through an inlet (not shown) so that powdercoating material entering the powder collection chamber 374 is fluidizedatop the porous plate 376.

In the presently preferred embodiment, two groups or banks of cartridgefilters 378 are located within the powder collection chamber 374 and arearranged in an inverted V shape as seen in FIG. 8. The open top of eachcartridge filter 378 is carried by one of the support plates 366, 367 ofclean air chamber 364 in position over an opening 368 in, such plates366, 367. Each cartridge filter 378 has a central rod 382 threaded atits upper end to receive a mount 384 which is tightened down on the rod382 such that one of the support plates 366 or 367 is sandwiched betweenthe mount 384 and the top of a cartridge filter 378. Preferably, one ormore filter mounting plates 386 extending between end walls 358, 360 ofcollector housing 350 provide additional support for each cartridgefilter 378.

In order to dislodge powder coating material from the walls of thecartridge filters 378, which enters the collector housing 350 asdiscussed below, a set or group of air jet nozzles 392 is provided foreach bank of cartridge filters 378. One set of air jet nozzles 392 iscarried on a nozzle support 394 mounted within clean air chamber 364,and the second set of air jet nozzles 392 is carried on a nozzle support396 within the clean air chamber 364. As depicted in FIG. 8, each set ofair jet nozzles 382 is aimed at the open tops of one group or bank ofcartridge filters 378. The air jet nozzles 392 associated with each bankof cartridge filters 378 are connected by air lines 398 to a pneumaticvalve 400, which, in turn, is connected to the source 73 of pressurizedair. In response to a signal from the system controller 108, thepneumatic valves 400 are operated to selectively direct pressurized airthrough air lines 398 so that a jet of pressurized air is emitted fromthe air jet nozzles 392 into the interior of one or both of the banks ofcartridge filters 378. These pulsed jets of air dislodge powder coatingmaterial from the walls of the cartridge filters 378 so that it can fallby gravity into the powder collection chamber 374 and onto the porousplate 376.

With reference to FIGS. 1 and 7, air entrained powder coating materialis drawn into each of the powder collection units 346 from the boothinterior 30 under the application of a negative pressure exerted by theblower units 348 mentioned above. Each of the blower units 348 includesa fan plenum 402 which houses a fan or blower 404 and a number of finalfilters 406 depicted schematically in FIG. 1. The fan plenum 402 isformed with a number of openings 408 over which an exhaust duct 410 isfixedly mounted. Each exhaust duct 410 extends vertically upwardly intoengagement with a coupling 412 located at the base of one of theextensions 373 of the clean air chambers 364 associated with each powdercollection unit 346. In response to the operation of blower 404 withinfan plenum 402, a negative pressure is developed within the exhaust duct410 and, in turn, within the clean air chamber 364 associated with eachof the powder collection units 346. This negative pressure creates adownwardly directed flow of air in the booth interior 30 within whichoversprayed powder coating material is entrained. The air entrainedpowder coating material passes through the gratings 38 at the floor 20of the spray booth 12 and enters each of the powder collection units 346where the powder coating material is collected along the walls of thecartridge filters 378 or falls onto the porous plate 376 at the base ofcollector housing 350.

An important aspect of the powder collection and recovery system 16 ofthis invention is that one blower unit 348 services a limited number ofpowder collection units 346. For example, the blower unit 348A depictedon the right hand portion of FIG. 7 has a fan plenum 402 formed withfour openings 408 each of which receive an exhaust duct 410 connected toone powder collection unit 346. Accordingly, four powder collectionunits 346 are accommodated by one blower unit 348A. Other blower units348 are associated with relatively small groups of adjacent powdercollection units 346 which results in the application of a uniform,downwardly directed flow of air throughout the booth interior 30.Further, the configuration of the clean air chamber extensions 373 ofeach powder collection unit 346 permits the powder collection units 346on one side of spray booth 12 to “dovetail” or fit closely adjacent thepowder collection units 346 on the opposite side of booth 12. See centerof FIG. 7. This conserves space and reduces the overall dimension of thebooth 12.

Another aspect of the powder collection and recovery system 16 of thisinvention is the retrieval of collected, oversprayed powder from thepowder collection units 346 for recirculation back to the powder kitchen14. As mentioned above, air entrained powder material from the boothinterior 30 is drawn into each of the powder collection units 346 andfalls either by gravity onto the porous plate 376 at the base thereof oris dislodged from the walls of the cartridge filters 378 by periodicbursts of pressurized air emitted from the air jet nozzles 392. In thepresently preferred embodiment, movement of the powder onto the porousplate 376 is assisted by the forming of the walls 354–362 of thecollector housing 350 of each powder collection unit 346 of a relativelythin gauge metal, such as 18–20 gauge No. 304 stainless steel, so thatthey vibrate when the reverse jets of pressurized air are emitted fromair jet nozzles 392. Because the porous plate 376 is angled at aboutfive degrees with respect to horizontal, the fluidized powder coatingmaterial thereon flows toward an outlet 422 on one side of the collectorhousing 350 at the lower end of porous plate 376. In turn, each of theoutlets 422 of powder collection units 346 is connected by a branch line424 to a common header pipe 426 which extends longitudinally along thelength of powder booth 12 on both sides thereof. The header pipe 426 isconnected to the reclaim line 86 which leads to the third powderreceiver 82 within, the powder kitchen 14. Preferably, a guillotine-typegate valve 428 is carried within each branch line 424, and these valves428 are movable between an open position to permit the flow of powdercoating material therethrough and a closed position to prevent suchflow.

In response to activation of the third vacuum pump 84 within the powderkitchen 14, which is associated with third powder receiver 82 andreclaim hopper 80 as described above, a negative pressure is producedwithin the header pipe 426. The system controller 108, mentioned abovein connection with the powder transfer system, is operative toselectively open the gate valves 428 associated with each powdercollection unit 346 so that the powder therein is drawn through theirrespective branch lines 424 into header pipe 426. Because of the largenumber of powder collection units 346, only a predetermined number ofgate valves 428 are opened at any given time to limit the total amountof powder material which is allowed to enter the header pipe 426 fortransfer to the reclaim line 86 leading to the third powder receiver 82and primary hopper 80.

With reference to FIG. 1, a pressure sensor 430 is schematicallydepicted as being connected to the fan plenum 402 of the blower unit348. The purpose of pressure sensor 430 is to sense the pressure dropacross final filters 406 within blower unit 348 and send a signalrepresentative of same to the controller 108. In the event of a failureor other problem with one or more cartridge filters 378 within thepowder collection unit 346 associated with a given blower unit 348, thepassage of powder coating material into the clean, air chamber 364 andthen to the final filters 406 creates a pressure drop across the finalfilters 406. This pressure drop is sensed by the pressure sensor 430 atwhich time a signal representative of such pressure drop is sent to thecontroller 108 to alert the operator of a problem within such powdercollection unit 346. Because there are a number of blower units 348,each associated with a group of powder collection units 346, a failurewithin the powder collection and recovery system 16 can be pinpointedand attributed to one blower unit 348 and an associated group of powdercollection units 346. This facilitates maintenance of the system andavoids the operator having to check each of the blower units 348 forsuch problems.

While the invention has been described with reference to a preferredembodiment, it should be understood by those skilled in the art thatvarious changes may be made and equivalence may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof.

For example, the system 10 of this invention has been depicted with asingle primary hopper 60, a single reclaim hopper 80, a feed hopper 68associated with a robot hopper 78 and robot 40, and, a feed hopper 96associated with an overhead gun manipulator 44. It should be understoodthat the embodiment of system 10 depicted in the FIGS. and describedabove is intended for purposes of illustration of the subject matter ofthis invention, and that the system 10 could be modified depending uponthe requirements of a particular application. Multiple primary hoppers60 and reclaim hoppers 80 can be employed, and a variety of spray gunconfigurations can be utilized including automatically and manuallymanipulated guns supplied with different combinations of feed hoppersand/or robot hoppers.

Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

1. An apparatus for applying powder coating material onto objects,comprising: a spray booth having a ceiling, floor, and opposed sidewalls defining an interior which provides a controlled area for theapplication of powder coating material onto objects moving therethrough;a powder collector located immediately adjacent said spray booth forcollecting oversprayed powder from said interior thereof, said powdercollector including: (i) powder collection units extending alongsubstantially the length of said spray booth, each of said powdercollection units having a powder collection chamber for receivingoversprayed powder from said interior of said spray booth; (ii) aplurality of clean air chambers, each of said clean air chambers beingassociated with one or more of said powder collection units, each ofsaid clean air chambers having an interior which is isolated from theoversprayed powder entering said powder collection chamber; (iii)filters located within said powder collector connected to said clean airchambers for collecting oversprayed powder thereon to form clean airwhich is introduced into said clean air chambers; a number of exhaustunits, each connected to a plurality of filters, for creating a negativepressure within said powder collector to draw air entrained powdercoating material from said spray booth into said powder collector,wherein each of said exhaust units includes a respective exhaust conduitand an exhaust fan, said exhaust conduit being connected by a pluralityof ducts to said clean air chambers, each of said ducts connected to onesaid clean air chamber, said exhaust fans being operative to create anegative pressure within said exhaust conduits, said plurality of ducts,said clean air chambers and said filters to draw oversprayed powder tosaid powder collector.
 2. The apparatus of claim 1 in which one or moreof said powder collection units is connected by a connector line to aheader pipe, each of said connector lines carrying a valve movablebetween an open position and a closed position.
 3. The apparatus ofclaim 1 in which each of said exhaust conduits includes a number offinal filters and means for sensing the pressure drop across said finalfilters.
 4. The apparatus of claim 1 in which said powder collectionchamber of each of said powder collection units includes a porous platemounted at an angle relative to horizontal in position to receive powdercoating material from said spray booth, said powder collection chamberbeing formed with an air inlet for directing a flow of air through saidporous plate to fluidize powder coating material received thereon. 5.The apparatus of claim 4 wherein said porous plate has a higher end anda lower end and wherein a connector line is connected to said collectionchamber near said lower end of said porous plate.
 6. The apparatus ofclaim 5 in which each of said connector lines of said powder collectionunits is connected to a header pipe.
 7. The apparatus of claim 1 inwhich each of said powder collection units is connected by a connectorline to a source of negative pressure to pull powder out of said powdercollection units through said connector line.
 8. The apparatus of claim7 further including a powder collection receptacle and a conduit forconnecting said connector lines to said powder collection receptacle. 9.The apparatus of claim 1 in which said powder collection chamber of eachof said powder collection units includes at least one porous platemounted at an angle relative to horizontal in position to receive powdercoating material from said spray booth, said porous plate having a lowend and a high end, said powder collection chamber being formed with anair inlet for directing a flow of air through said porous plate tofluidize powder coating material received thereon, a connector linebeing connected to said collection chamber near said low end of saidporous plate.
 10. The apparatus of claim 9 in which each of saidconnector lines of said powder collector units is connected to a sourceof negative pressure to pull powder out of said powder collection unitsthrough said connector lines.
 11. An apparatus for applying powdercoating material onto objects, comprising: a spray booth having a floor,ceiling and opposed side walls defining an interior which provides acontrolled area for the application of powder coating material ontoobjects moving therethrough; a number of powder collection units locatedimmediately adjacent said spray booth for collecting oversprayed powderfrom said interior thereof; a control element for sequentiallytransferring the oversprayed powder coating material, under theapplication of negative pressure, from said powder collection units to aheader pipe; a suction device for transferring the oversprayed powdercoating material, under the application of negative pressure, from saidheader pipe to a hopper.
 12. The apparatus of claim 11 in which each ofsaid powder collection units has a connector line, two or more of saidconnector lines being connected to said header pipe, said control meansincluding a valve mounted with respect to each of said connector linesto control the flow of powder coating material therethrough.
 13. Theapparatus of claim 11 in which each of said powder collection unitsincludes a powder collection chamber, each of said powder collectionchambers including a porous plate mounted at an angle relative tohorizontal in position to receive powder coating material from saidspray booth, said powder collection chambers each being formed with anair inlet for directing a flow of air through said porous plate tofluidize powder coating material received thereon.
 14. The apparatus ofclaim 13 wherein said porous plate has a higher end and a lower end andwherein a connector line is connected to said powder collection chambernear said lower end of said porous plate.
 15. The apparatus of claim 14in which each of said connector lines of said powder collection units isconnected to a source of negative pressure to pull powder out of saidpowder collection units through said connector lines.
 16. The apparatusof claim 13 in which each of said powder collection units is connectedby a connector line to a source of negative pressure to pull powder outof said powder collection units through said connector line.
 17. Theapparatus of claim 16 further including a control element forselectively pulling powder out of said powder collection units throughsaid connector lines.
 18. The apparatus of claim 16 further including apowder material receptacle and a conduit for connecting said connectorlines to said powder material receptacle.
 19. The apparatus of claim 13in which said powder collection chamber of each of said powdercollection units includes at least one porous plate mounted at an anglerelative to horizontal in position to receive powder coating materialfrom said spray booth, said porous plate having a low end and a highend, said powder collection chamber being formed with an air inlet fordirecting a flow of air through said porous plate to fluidize powdercoating material received thereon, a connector line being connected tosaid collection chamber near said low end of said porous plate.
 20. Theapparatus of claim 19 in which each of said connector lines of saidpowder collector units is connected to a source of negative pressure topull powder out of said powder collection units through said connectorlines.
 21. The apparatus of claim 20 further comprising a controlelement for selectively pulling powder out of said powder collectionunits through said connector lines and for depositing said powder in apowder material receptacle.
 22. The apparatus of claim 21 wherein saidconnector lines are connected to said common header pipe and said powdermaterial receptacle.